hello I'm Jim Salsbury with Mitutoyo America Corporation and welcome to the
Metrology Training Lab. in this episode we're going to talk about the
calibration of calipers in past episodes we've defined the concept of calibration
discussed the purpose of calibration and introduced important aspects of what
makes a good calibration method in this episode we'll apply all those ideas to
the calibration method for a caliper calibration is a search for airs and we
want to develop a calibration method that efficiently hunts down and finds
any potential errors in the measuring instruments so how do we best do that
for a caliper well there's a nice American national standard for digital
dial and vernier calipers called ASME B89.1.14 and
the calibration method that we'll be discussing is based on that standard
calibration usually starts with checking out the proper operation of the caliper
is there any obvious damage does it move smoothly do the buttons work correctly
and once you're satisfied with the operation then we begin the accuracy
tests on calipers the conditions of the measuring jaws over time is always a
concern for the external measuring faces before you begin measuring anything you
bring the measuring faces close together but not touching and then look at the
gap between them in front of a light
this may look silly but it actually works the use of the light gap is an
efficient way to see anywhere or bending of the jaws if you see something
you can then do a more thorough and quantitative test by using a gauge pin
this is done by simply moving the gauge pin from near the beam to the tips of
the jaws while observing the readings
here all the values are zero so we know that the flatness and parallelism of the
jaws are in good shape the use of a light gap is also good for checking the
condition of the internal measuring faces in particular when you have the
crossed knife edge type like we have on this caliper to check these internal
measuring jaws you don't create a gap but instead you close the calipers and
then slightly rotate it and look at that gap at an angle to check the condition
of the measuring faces again you might think I look silly and looking at gaps
of light might not feel too scientific but it has been proven over and over to
work pretty well the very common caliper like this one here has four different
types of measurements it can make and therefore our calibration method needs
to be properly engineered to sufficiently and most efficiently hunt
down the potential errors as already mentioned this caliper can make both
outside and inside measurements most calipers like this up to 12 inches also
have some type of depth bar for making measurements into holes or slots the
fourth type of measurement that's possible with this caliper is the step
measurement which uses these two phases to make measurements like this step
height on this cylindrical workpiece this type of caliper is quite common so
we will discuss the necessary accuracy tests to calibrate this caliper
there are other configurations of calipers out there and after watching
this episode you should be able to extend what we discuss to the
calibration of all types of calipers for all of our measurements we will want to
properly use the caliper the most important skill and calibrating calipers
is simply using it correctly if you're having some struggles getting good
results find yourself a caliper that's known to be in good condition
grab some reference standards and practice it doesn't take too long to get
the right field the general principle of the calibration method which is based on
the requirements of the ASME be 89 114 standard is that all four measurement
methods outside-inside depth and step all use the same measuring scale in
addition the zero points is only set once based on the outside measuring
faces and so from all that the calibration method is to measure
multiple points across the measuring range using the outside measuring faces
and then only a single test point with each of the other measuring faces now
let's look at this method more carefully first we will clean all the measuring
faces and then bring the outside measuring faces together to set our zero
we will double-check our zero but when we're done with that we won't change it
again during the calibration the first test is with the outside measuring faces
for 6-inch caliper the ASME standard requires a minimum of three points
across the measuring range the minimum then increases for larger calipers we
will take four as that is convenient for the reference standard that we're using
which is this military caliper checker this is a nice standard for calibrating
calipers but you can use any grade or material gauge block as well well start
at one inch in accordance to the ASME standard the length specifications apply
at any point across the measuring faces thereby including any influence of the
jaw parallelism during the calibration it's important to intentionally vary the
distance from the beam at different test points at the 1-inch points I'll start
close to the beam and report that value I'll also check it further out just to
see if the caliper may give me some problems when using or calibrating a
caliper it's important to learn how to get a consistent value the amount of
force you apply can vary the results as you see here I am applying too much
force and the values changed by several thousandths of an inch so don't do that
alignment is also a problem if you're misaligned in this direction you can see
the values get larger if you are misaligned in this direction again the
values get larger
so you want to rock the caliper in both directions while applying a consistent
force and looking for the smallest value
it looks like the air at this test point is minus 5/10 so I'll write that down
it's understood that when using a caliper you need to rock it around a bit
to get the proper result this applies to normal use as well as in calibration
it's not correct however to take a bunch of readings and report the average
values during the calibration the caliper should be repeating within the
specification limits for the length accuracy and if not and the caliper
should be rejected it is also not correct to rock it around until you get
the value you like right this digital caliper has a resolution in inches of
1/2 a foul or 5/10 in metric the resolution is 10 microns and if you
don't understand a thigh or 1/10 or microns go watch our episode of the
metrology training lab but just discusses the units of measure for this
size digital caliper the tolerance is plus or minus one thousandths of an inch
for all the test points that we're taking including the outside inside
depth and step caliper specifications can be tricky to understand I recommend
you to review the technical Bolton we've put together on this topic you can find
that on our on-demand educational websites so let's continue with the
other test points at 2 inches I'll measure about halfway down from the beam
to the tip to the jaws after some rocking around the air looks to be minus
5/10
moving to four inches
the air also looks to be minus five tenths at six inches I'm going to
measure out near the tips of the jaws I need to be very careful here with my
force
I'm applying too much force there and that feels better after a little
wiggling around once again the air looks to be minus five tenths
this completes the testing of the outside measuring phases an alternative
to the caliper checker is to use gauge blocks that's fine but be careful you
need to measure them like this because you need to move across the jaws like we
showed with the caliper checker you can't measure them like this you want to
ensure your test method is sensitive to the parallelism of the jaws it's
tempting to measure the gauge blocks across the entire measuring faces that's
easier or maybe to measure them only near the beam that's usually less
influenced by your force however it's not our job to calibrate the caliper and
only its easiest or best mode of use on the contrary we need to assess its
accuracy across any reasonable and likely use now let's move to the inside
measuring faces and we will use this caliper checker again if you don't have
a caliper checker a ring gauge like this is a good solution as well as mentioned
before you only need a single test points and the ASME standard states the
size to be somewhere between three quarters of an inch and two inches the
technique for inside measuring faces is a little more difficult if you rock this
way the values get smaller again but if you rock this way
they get larger so you need to wiggle this way
looking for the smallest value all while applying a constant force
and then we go this way looking for the largest value looks like I'm getting
minus five tenths
this caliper has crossed knife edge inside measuring faces they are sharp
but of course there is still some thickness to these edge faces and
there's some gap between the two edges it's pretty well known that if you
measure a small inside diameter that you may measure undersized when using the
cross knife edge measuring faces this is particularly a problem when there are
some damage to the faces so it's valuable to check this during
calibration this is done by using a small ring gauge like this ceramic
setting ring the ASME standard specifies a point two inch diameter for this test
like we have here the measuring this ring gauge is pretty easy just apply a
good consistent force and then wiggle it around a little bit looks like I'm
measuring minus a thousandth of an inch
for the last two measurements in the calibration we'll use a gauge block
placed on this surface plate for both the depth and step the ASME standard
specifies a single test point less than two inches so we'll use this one inch
gauge block here metoya we like square blocks for the depth
measurements for obvious reasons in this case I measure an air plus 5/10 so we'll
write that down lastly we do the step measurements again a square gaze block
works well we'll use the same one-inch gauge block we can hold the caliper
against the block and simply take our reading looks like the air is minus a
thousandth of an inch so we'll write that down
so this completes the calibration all the airs here are within the plus or
minus one thousandth of an inch specification so it looks like this
caliper is in tolerance and that's the calibration of calipers
remember the method we just discussed comes from the American national
standard on calipers asme be eighty nine point one point fourteen lots of experts
from around the country helped write this documents but the b89 committee is
always looking for ideas to improve the standards minatoya America is heavily
involved with this committee and if you would like to post comments on this
standard good or bad we gladly bring them to the committee's attention the
standards committees are also open to the public and you're welcome to attend
thank you I'm Jim Salsbury and I'll see you next time
from the metrology training lab